1. Field of the Invention
This invention relates to a solder paste screen printing apparatus and a solder paste screen printing method for coating solder paste, used to solder electronic parts, onto a workpiece such as a substrate, a chip and a wafer.
2. Description of the Related Art
In a screen printing apparatus for coating solder paste, used to solder electronic parts, on a workpiece such as a substrate, a screen mask is superposed on the substrate, and in this condition a squeegee is slid over the screen mask, so that the solder paste is coated onto electrodes on the substrate through pattern holes formed through the screen mask.
If the force of pressing of a lower end of the squeegee against the upper surface of the substrate is small, the solder paste is not completely filled in the pattern holes, so that the printing becomes defective. In contrast, if the pressing force is too large, the solder paste flows to the lower surface of the screen mask, thereby causing the bleeding, so that the defective printing is encountered. The finer the pitch of the electrodes becomes, the more serious this bleeding problem becomes. In view of these, the squeegee-pressing force must be adjusted to the optimum value.
Under the circumstances, there has been proposed a method of enabling the delicate adjustment of the pressing force (Japanese Patent Unexamined Publication No. 7-164615). In this method, a compression torsion coil spring imparts a spring force acting in a direction to float a squeegee, thereby reducing the force of pressing of the squeegee against a screen mask, so that the required pressing force can be imparted by a cylinder. Therefore, in this method, particularly with respect to electrodes of a fine pitch on the substrate, the pressing force is reduced to a level less than the weight of the squeegee, so that solder paste can advantageously be printed onto the electrodes with a very small pressing force without bleeding.
In the above conventional method, however, the solder paste must be repeatedly coated for trial purposes in order to adjust the spring force of the compression torsion coil spring, and therefore there has been encountered problems that much time and labor are required for the preparations, and that the differences (or variations) between the individual operators are liable to occur.
And besides, the substrate has a warp and undulations, and therefore the squeegee, when slid, moves up and down in accordance with these warp and undulations, and as a result the spring force or the spring constant of the compression torsion coil spring varies in accordance with this upward and downward movement, and the pressing force also varies, so that the printing is liable to become defective.
Furthermore, since it is difficult to measure the pressing force during the time when the squeegee is slid over the screen to effect the printing operation, it is not possible to know the degree of the pressing force with which the printing is actually effected, and therefore there has been encountered a problem that the delicate adjustment of the pressing force is difficult.